--- provision-promote-W8TQ3MZC.ps1
+++ provision-promote-R2VK8FQN.ps1
@@ -33,7 +33,12 @@
     return [int]$value
 }
 function Assert([bool]$Condition, [string]$Name) {
-    if (-not $Condition) { throw $Name }
+    if (-not $Condition) {
+        $exception = [InvalidOperationException]::new($Name)
+        $exception.Data['assertion_name'] = $Name
+        $exception.Data['source_line'] = (Get-PSCallStack)[1].ScriptLineNumber
+        throw $exception
+    }
 }
 function Full([string]$Path) {
     if ($Path.StartsWith('\\?\UNC\', [StringComparison]::OrdinalIgnoreCase)) { $Path = '\\' + $Path.Substring(8) }
@@ -61,6 +66,43 @@
 }
 function Read-Json([string]$Path) {
     return ConvertFrom-Json -InputObject ([IO.File]::ReadAllText($Path, $Utf8)) -AsHashtable -DateKind String
+}
+function Read-SharedText([string]$Path) {
+    $file = [IO.FileStream]::new($Path, [IO.FileMode]::Open, [IO.FileAccess]::Read,
+        ([IO.FileShare]::ReadWrite -bor [IO.FileShare]::Delete))
+    try {
+        $reader = [IO.StreamReader]::new($file, $Utf8, $true)
+        try { return $reader.ReadToEnd() } finally { $reader.Dispose() }
+    } finally { $file.Dispose() }
+}
+function Read-PromotionLog([string]$Path, [bool]$RolledAbsentBeforeRun) {
+    $current = Read-SharedText $Path
+    if ($RolledAbsentBeforeRun -and [IO.File]::Exists("$Path.1")) {
+        return (Read-SharedText "$Path.1") + "`n" + $current
+    }
+    return $current
+}
+function Has-OwnedPromotion([string]$Log, [long]$BrainPid, [long]$Generation, [string]$Marker) {
+    $header = '=== spt brain stderr — generation ' + $Generation + ' — pid ' + $BrainPid + ' ==='
+    $headers = [regex]::Matches($Log, '(?m)^=== spt (?:brain|broker) stderr — generation \d+ — pid \d+ ===\r?$')
+    for ($i = 0; $i -lt $headers.Count; $i++) {
+        if ($headers[$i].Value.TrimEnd("`r") -cne $header) { continue }
+        $start = $headers[$i].Index + $headers[$i].Length
+        $end = if ($i + 1 -lt $headers.Count) { $headers[$i + 1].Index } else { $Log.Length }
+        if ($Log.Substring($start, $end - $start).Contains($Marker)) { return $true }
+    }
+    return $false
+}
+function Get-FailureDiagnostic([Management.Automation.ErrorRecord]$Record) {
+    $cause = $Record.Exception.GetBaseException()
+    $name = $null
+    $line = $Record.InvocationInfo.ScriptLineNumber
+    if ($cause.Data.Contains('assertion_name')) {
+        $name = [string]$cause.Data['assertion_name']
+        $line = [int]$cause.Data['source_line']
+    }
+    return @{ assertion_name = $name; exception_type = $cause.GetType().FullName;
+        hresult = $cause.HResult; source_line = $line }
 }
 function New-Bytes([string]$Path, [byte[]]$Bytes) {
     $stream = [IO.FileStream]::new($Path, [IO.FileMode]::CreateNew, [IO.FileAccess]::Write, [IO.FileShare]::Read)
@@ -440,6 +482,9 @@
 
     $Failure = 'OWNED_PROMOTION_READINESS_FAILED'
     Require-Time 25
+    $DaemonLogPath = Join-Path $H 'logs/daemon.stderr.log'
+    # A backup absent before this owned launch can only enter evidence after this run starts.
+    $RolledAbsentBeforeRun = -not [IO.File]::Exists("$DaemonLogPath.1")
     $Run = Start-Native 'promotion-daemon' $E @('daemon', 'run') @{ SPT_HOME = $H; SPT_INSTALL_NO_FIREWALL = '1' } 180 'run'
     # This launcher is a separate run-scoped process, never a descendant of the apply/readiness step.
     $readySeconds = Cap 60 20
@@ -473,12 +518,13 @@
     do {
         Require-Time 15
         Assert (-not $Run.process.HasExited) 'RUN_EXITED_BEFORE_PROMOTION'
-        $Log = [IO.File]::ReadAllText($Run.stderr_file, $Utf8)
+        $Log = Read-PromotionLog $DaemonLogPath $RolledAbsentBeforeRun
         Assert ($Log -notmatch 'PROMOTE_BYTES_UNVERIFIED|BRAIN_PROMOTE_REJECTED|BRAIN_ROLLBACK') 'PROMOTION_BYTES_UNVERIFIED_OR_ROLLBACK'
         $Applied = Read-Json $StatePath
         Assert ($Applied.phase -cin @('applied-pending', 'applied')) 'PROMOTION_STATE_REFUSED'
         $marker = 'BRAIN_PROMOTED: candidate v' + $N + ' signalled ready (generation ' + $BrainReady.generation + ')'
-        if ($Applied.phase -ceq 'applied' -and (Numeric-N $Applied.version) -and $Log.Contains($marker)) { break }
+        if ($Applied.phase -ceq 'applied' -and (Numeric-N $Applied.version) -and
+            (Has-OwnedPromotion $Log $Membership.brain.pid $BrainReady.generation $marker)) { break }
         Assert ((Now) -lt $PromotionEnd) 'DURABLE_PROMOTION_WAIT_EXHAUSTED'
         Start-Sleep -Milliseconds 100
     } while ($true)
@@ -590,6 +636,7 @@
     Write-Output 'PROMOTION_COMPLETE=YES'
 }
 catch {
+    $FailureDiagnostic = Get-FailureDiagnostic $_
     # Never print exception messages, environment values or unqualified path/error payloads.
     # On any failure the same owned run stop channel is the sole shutdown request.
     if ($null -ne $Run) {
@@ -607,11 +654,13 @@
     }
     if ($OwnOutput) {
         try { New-Json (Join-Path $OutputRoot 'promotion-failure.json') @{ accepted = $false; error = $Failure;
+            failure = $FailureDiagnostic;
             shutdown = $Shutdown; deadline = $Deadline; recorded_utc = [DateTime]::UtcNow.ToString('o');
             native_records = @($Owned | ForEach-Object { $_.record_file }); public_partials_preserved = $true;
             rig_disposal_performed = $false; product_applied_state_written_by_instrument = $false } } catch { }
     }
     [Console]::Error.WriteLine($Failure)
+    [Console]::Error.WriteLine((ConvertTo-Json -Compress -InputObject $FailureDiagnostic))
     exit 1
 }
 finally {